Coordinates

The UCD are the same whether the values are sexagesimal or decimal values.

Note that both UCDs could be complemented with the ;meta.main qualifier to indicate that the columns are the main values, but as there is only one right ascension and declination in the table, this is not mandatory (unless this is explicitly required by some protocol).

In this case, there are two right ascensions (and declinations), and so it can be useful to append ;meta.main to one of the values: this will indicate that if a tool has to pick one right ascension value, the column described by pos.eq.ra;meta.main is to be preferred ( alpha2 here). Therefore, one should avoid to attach identical UCDs containing meta.main to several column within a single table, as it would create ambiguities.

The positions in pixel values are described as cartesian coordinates. We use the secondary word instr.det to indicate the kind of frame these coordinates are related to. Note that the instr.det is optional: the first word carries most of the meaning.

The position offset in pixel is a difference between two coordinates that would be described by the same UCD primary word, so we use the arith.diff word to describe it.

We considered here that the coordinates of the photocenter were the main coordinates to use for each source.